Formation Of Mullite Whiskers Research Articles

Sintering inhibition and oil-absorption properties of calcined coal-series kaolin

AbstractIn this study, aluminium fluoride trihydrate (AlF3⋅3H2O) was used to inhibit the sintering of calcined coal-series kaolin (CCSK). In addition, the oil absorption properties of CCSK were studied. The particle-size distribution, specific surface area and porosity of the samples were investigated as a function of calcination temperature and the addition of AlF3⋅3H2O. Moreover, the ability of AlF3⋅3H2O to improve the oil absorption of CCSK was explored. The morphology, structure and phase composition of the specimens were investigated using scanning electron microscopy and X-ray diffraction. The phase transition during heating of the samples was studied using thermogravimetric analysis. The preparation with 10 wt.% AlF3⋅3H2O had the optimal sintering inhibition effect on CCSK at 1000°C. The release of SiF4 gas during heating and the formation of mullite whiskers on the particle surface caused by AlF3⋅3H2O moderated the formation of molten-phase liquid bridges between particles and inhibited sintering of the raw material powder. Furthermore, the formation of submicron mullite whiskers on the surface of the CCSK particles enhanced the oil-absorption properties of the sample significantly.

Mechanical properties of large-sized thin architectural ceramic plate enhanced by alumina fibres and in situ mullite whiskers

The fibres reinforced thin architectural ceramic plates of 900?1800?2.5mm with excellent mechanical properties were prepared by fast-sintering method using a controllable fibre dispersion process. The effects of ball-milling time on dispersity, average length-to-diameter ratio and microstructure of alumina fibres were investigated. Meanwhile, the effects of alumina fibre contents on the bulk density, water absorption, phase transformation and microstructure of the thin ceramic plate were researched. It was found that the two-step ball-milling process can effectively control the average length-to-diameter ratio of alumina fibres, achieving a good dispersion mixture of fibres and ceramic powders. Ceramics bulk density and bending strength increase with fibre contents rise from 0 to 5 wt.% and then decrease with further fibre content addition from 5 to 15wt.%. The in situ formed mullite whiskers via fast-sintering method are beneficial for protecting fibres and fibre/matrix interfaces. The maximum value of bending strength and fracture toughness reach 147MPa for 5 wt.% fibre contents and 2.6MPa?m1/2 for 9 wt.%fibre contents, corresponding to the strengthening of alumina fibres and the formation of mullite whiskers in fibre/matrix interfaces and matrix via fast-sintering process.

Novel facile nonaqueous precipitation in-situ synthesis of mullite whisker skeleton porous materials

High strength, lightweight mullite whisker skeleton porous materials were prepared via novel facile nonaqueous precipitation method. The phase transition process of precipitate, microstructures and phase compositions of samples, and effects of aluminum fluoride amount on morphologies and properties of mullite whisker skeleton porous materials were investigated by DTA-TG, FE-SEM, XRD, the acid solubility and compressive strength tests. The growth direction of the whisker was determined. The growth mechanism of mullite whisker and formation mechanism of skeleton porous material were also investigated. The results show that the mullite phase forms at a low temperature of 900 °C via nonaqueous precipitation method, due to the formation of Al-O-Si bonds in the precipitate. The formation of mullite whiskers is controlled by gas-solid reaction and the whiskers grow along [001] direction. Well-developed mullite whiskers interlacedly grow into skeleton porous material at 1100 °C. The optimum mullite whisker skeleton porous material with the optimized aluminum fluoride amount of 1.3 of theoretical amount exhibits attractive properties of low density (1.21 g cm−3), high strength (158.6 MPa), low acid solubility (1.58 wt%), low thermal expansion coefficient (1.6 × 10−6 °C−1) and low thermal conductivity (0.28 W/(m K)). This material has the mean pore size of 542 nm and pore size distribution of 32–865 nm. It shows attractive application prospects in filtering and biomedical applications.

Effect of AlF<sub>3</sub> Content on Microstructure and Properties of Mullite/Al<sub>2</sub>O<sub>3</sub> Composite Ceramics

To reinforce the mullite/Al2O3 composite ceramics through formation of mullite whiskers, the composite ceramics were prepared by pressureless sintering using different AlF3 content. The microstructure, porosity, fracture toughness and thermal shock resistance of the composite ceramics were investigated. The results show that the addition of AlF3 can promote the mullite whisker formations, and the whiskers with the size of 3～10μm in diameter and a length-diameter ratio of 10～15 are obtained by sintering at 1600°C with the AlF3 content of 5wt%. Fracture toughness and thermal shock resistance of the composite ceramics are improved by the formation of mullite whisker. The fracture toughness of 4.79MPa•m1/2 can be obtained, and the 95.18% flexural strength remained after thermal shock.

Fabrication and Property of Mullite Whiskers Frameworks with an Ultrahigh Porosity by Expandable Mesocarbon Microbeads

Mullite whiskers frameworks with an ultrahigh porosity were fabricated by the vapor‐phase reaction of AlF3, Al2O3, and SiO2 and adding expandable mesocarbon microbeads (MCMB) as a pore‐forming agent. A large volume expansion of 122% for MCMB due to its layered structure occurred during the formation of mullite whiskers, resulting in the expansion of samples and high porosities of 87.7%–98.2% at 50–90 wt% MCMB contents. Perfect whiskers and a lap‐joint structure formed due to the formation of mullites through the vapor‐phase reaction. A bimodal pore structure was achieved from the spaces of the whiskers framework and burning of the expanded MCMB. High compressive strengths of 1.7 to 5.4 MPa were obtained for the porous mullite at porosities of 94.2%–87.7%, which suggested a rigid structure; these strengths at the ultrahigh porosities are attributed to the merit of the framework with high strength whiskers and their strong bonding.

Preparation of Mullite Whiskers from Kaolinite Using CuSO<sub>4</sub> as Fluxing Agent

Mullite whiskers were produced by firing of kaolin at 1400o-1550oC. CuSO4 was used as fluxing agent. Before firing, spray drying and baking with flour and leaven were employed to give free space to the powders. The influence of preparation parameters and firing temperature on the crystalline structure and phase transformations during the entire process was determined. The results showed that there was a beneficial effect of both the incorporation of CuSO4 and the given free space on the formation of mullite whiskers. There was no evidence for the effect of firing temperature on the aspect ratio and morphology of the whiskers within the investigated range.

Growth of Mullite Whiskers on Alumina Particle by Thermal Decomposition of Clay Minerals

Whisker formation on alumina particles was attempted through growth of a number of fine mullite whiskers by the thermal decomposition of clay for porcelain at 1400 to 1600°C. After firing, the excess silica phase was hydrothermally dissolved in 2N NaOH solution at 180°C. The effect of firing temperature, and compositions of alumina and clay on the formation of mullite whiskers on alumina particles was investigated. In case of 18.8mass% alumina and 81.2mass% calcined clay, the growth of whiskers enhanced by the decomposition of clay was promoted, and alumina particles were coated with a number of mullite whiskers 50 to 200μm in length and 1 to 3μm in diameter just like sea urchin. Furthermore, alumina particles reacted with the excess amorphous silica in the fired clay to form small pillarlike mullite crystals in an alumina particle. But growth of whiskers was inhibited with increasing amount of alumina.